The present invention, in some embodiments thereof, relates to primary lithium electrochemical cells more particularly, but not exclusively, to primary lithium cells including a combination of a liquid SO2 cathode and an electrochemically compatible solid cathode.
There are several main commercial primary lithium electrochemical cells in market: Lithium Thionyl chloride (Li/SOCl2), Lithium Sulfuryl chloride (Li/SO2Cl2), Lithium Sulfur-dioxide (Li/SO2), Lithium Manganese dioxide (Li/MnO2), Lithium carbon monofluoride (Li/CFX) and Lithium Iron sulfide (Li/FeS2). A metallic lithium anode is common in all the above systems while the cathode material is different. The first three systems are known as liquid cathode systems while the last three systems are known as solid cathode systems.
In the liquid cathode systems the cathode material is in liquid state. During the cell's discharge the liquid cathode transfers the electricity to a porous high surface area conductive current collector. In the solid cathode systems, the cathodic active material is usually in the form of a solid that is mechanically attached to the current collector to conduct the current.
The electromotive force (EMF) of these systems is markedly different. The Li/SO2Cl2 system has an EMF of about 3.9V. The Li/SOCl2 system has an EMF of about 3.7V. The Li/MnO2 and the Li/CFX systems have an EMF of about 3.1V. The Li/SO2 has an EMF of about 3.1V. The Li/FeS2 system has an EMF of about 1.8V
In all the above mentioned lithium systems, during cell discharging, electrons are transferred in the external circuit from the negative pole of the cell to the positive pole. The anode is oxidized to lithium ions while the cathodic material is reduced and changes its valence to a lower state. Inside the cell, the lithium ions move from the anode side to the cathode side to naturalize the charge.
For example, in the Li/MnO2 system, during discharging of the cell, electrons are transferred from the lithium anode through the external circuit. The electrons are transferred through the positive pole of the cell to the manganese dioxide cathode to reduce the manganese which changes its valence from +4 to +3. Inside the cell, the lithium metal is oxidized to lithium ions that move within the electrolyte filling the cell and penetrate the cathode to balance the charge of the reduced manganese.
A similar type of mechanism occurs in the liquid cathode primary systems. For example, in the Li/SO2 liquid cathode system, during the discharging of the cell, electrons are transferred from the lithium anode through the external circuit (the load). The electrons are transferred through the positive pole of the cell to the SO2 cathodic material and reduces the SO2 to S2O4−2 ions, changing the valence of sulfur from +4 (in SO2) to +3 (in S2O4−2) Lithium ions move from the anode side to the cathode side and combine with S2O4−2 ions to form solid Li2S2O4 that is deposited on the porous current collector.
The electrolyte of the Li/SO2 and the Li/MnO2 systems contains lithium salts that are dissolves in an organic solvent or a mixture of organic solvents to form a conducting solution that conducts the electricity inside the cell. For the Li/SO2 cell, the electrolyte typically includes acetonitrile (AN) as a solvent and Lithium bromide (LiBr) as the ionizable salt. In the Li/MnO2 system, the electrolyte may typically contains propylene carbonate (PC) as the solvent and lithium perchlorate (LiClO4) as the conducting salt.
In Li/SO2 liquid cathode cells, the cathode active material (SO2) is dissolved in the AN solvent while in the Li/MnO2 cell the solid manganese dioxide is blended as solid inside a porous current collector.
As lithium metal is a very reactive material, it may spontaneously react with the liquid inside the cell. The metallic lithium may react either with the solvent of the electrolyte solution or with the liquid cathode material (SO2). For example, during the production process of a Li/SO2 cell, as soon as the liquid cathode solution is injected into the cell, the metallic lithium anode spontaneously reacts with SO2 to form a Li2SO2O4 film that is precipitated on the anode and is known as a solid electrolyte interphase (SEI). The SEI prevents further reaction of the SO2 with the lithium anode.
Similarly, as the electrolyte is injected into a cell of the solid cathode systems, metallic lithium of the anode may react with the PC solvent to form an insoluble Li2CO3 passivation layer on the anode that prevents further reaction of the lithium with the solvent.
In contrast to the Li/MnO2 system, the SEI of Li/SO2 cells is formed by the reaction of lithium with the SO2 and not with the AN solvent. The reaction of lithium with SO2 is thermodynamically preferred over the reaction between lithium and the solvent AN. In the absence of SO2 (such as for example, in unbalanced Li/SO2 cells having an excess of lithium in the anode, when the cell is fully discharged) a reaction of lithium with AN may forms toxiclithium cyanide (LiCN) and hydrogen cyanide (HCN) gas-. Therefore, excess of SO2 is usually required.
The electrical capacity ratio of lithium metal to SO2 has to be kept in an adequate manner. When the molar ratio of lithium to SO2 is above unity (Li/SO2>1.0) at the end of discharge, all SO2 is consumed and a reaction between lithium metal and AN may result leading to formation of LiCN and HCN gas that may rupture the cell. Due to this limitation SO2 must be in excess and the capacity of the cell is limited to a certain value depending on cell size.
The primary Li/SO2 system is a relatively a mature technology. The approximate capacity of a typical standard D size Li/SO2 cell is about 7.5 Ah. Despite some improvements in cell properties, this charge capacity remained almost the same for the last 30 years.